The present invention relates to a solid electrolytic capacitor and a method of producing the same.
An already known solid electrolytic capacitor has a configuration constituted by an anode, a dielectric layer, an electrolyte layer and a cathode, and is generally obtained by forming, on a metal having a valve action (valve-action metal) and constituting an anode, an oxide film as a dielectric layer (hereinafter called a dielectric layer), then forming thereon a solid electrolyte layer as a semiconductor layer and further forming a cathode member of, for example, graphite.
The valve-action metal is a metal capable of forming an oxide film of a controllable thickness by anodizing, and can be Nb, Al, Ta, Ti, Hf, Zr etc., among which Al and Ta are principally utilized for the practical purposes.
Of these, Al is employed as the anode in a form of an etched foil, while Ta is employed as the anode in a form of a porous material formed by powder sintering.
An electrolytic capacitor of the porous sintered type can be with a particularly small size and a large capacity relative to other solid electrolyte capacitors, and is in strong demand as a component capable of enabling reduction in size of a mobile telephone a portable information terminal equipment etc.
For example, a known solid electrolytic capacitor utilizing Ta is obtained by forming a dielectric layer on a surface of an anode member formed by sintering a powder mixture in which a lead wire is embedded, and then forming, on the aforementioned dielectric layer, a conductive polymer layer, containing for example carbon powder, as a solid electrolyte layer.
On the electrolyte layer formed as a semiconductor layer, a graphite paste layer and an Ag paste layer are formed which function as a cathode.
Then lead frames are connected respectively to the lead wire of the anode member and the Ag paste layer, and the entire structure is resin molded so as to expose these lead frames.
In such a known solid electrolytic capacitor, the electrolyte layer includes many cavities, into which conductive particles constituting the cathode member formed on the electrolyte layer penetrate.
Such a phenomenon, when present extensively, provides an advantage of lowering an equivalent serial resistance (ESR) of the solid electrolytic capacitor itself, and also provides an advantageous effect of securing a capacitance even at a high frequency.
However, in case the cathode-constituting material (conductive material) penetrating into the cavities of the electrolyte layer reaches a defect on the dielectric layer, a concentration of an electric field takes place in the area of the defect so as to cause heat generation or crystallization of the dielectric layer, thereby eventually leading to a breakdown of the dielectric layer.
For this reason it has been desired, in a solid electrolytic capacitor, to reduce a leak current by preventing a dielectric breakdown resulting from the deposition of the aforementioned cathode-constituting particles to a defect formed in the dielectric layer, without hindering the penetration of the cathode-constituting particles into the electrolyte layer (hereinafter called a first electrolyte layer).
In order to meet such a requirement, Japanese Patent Application No. 2001-359779 (hereinafter called prior technology 1) proposes a solid electrolytic capacitor including an anode member which is formed by sintering powder of a valve-action metal and in which a lead wire is embedded, a dielectric layer formed on a surface thereof, an electrolyte layer formed on the dielectric layer, a cathode member formed on such first electrolyte layer, and a silver paste layer formed on the cathode member. The solid electrolytic capacitor is completed by respectively connecting external terminals with the lead wire and the silver paste layer, and resin molding the entire configuration so as to expose such external terminals. The electrolyte layer includes particles constituting the cathode member, and non-conductive particles are made to be present between the dielectric layer and the electrolyte layer.
Such prior technology 1 is proposed to solve the aforementioned drawbacks, but is confirmed to elevate the ESR considerably by the aforementioned process and is therefore difficult to apply to a solid electrolytic capacitor requiring a lower ESR property.